Acidity of substituted MCM-41-type mesoporous silicates probed by ammonia

The acid properties of Al-, Ga- and Fe-substituted mesoporous silicates have been investigated using ammonia as a probe molecule. Substitution leads to the formation of Bronsted and Lewis acid sites of different type and strength. The IR spectra of ammonia-loaded samples show bending vibration bands near 1450 cm(-1) (ammonium ions at Bronsted sites) and at ca. 1620 and 1300 cm(-1) (ammonia located at weak and strong Lewis sites, respectively). The vibration bands of bridging Bronsted acid hydroxy groups appear at 3605 (Al) and 3591 cm(-1) (Ga). Initial heats of ammonia chemisorption decrease in the order: 185 (Al), 162 (Ga) and 144 (Fe) kJ mel(-1). The degree of isomorphous substitution of framework silicon by trivalent atoms decreases in the order Al >> Ga > Fe. The ratio of strong to medium-strong Bronsted sites falls in the same order. The strength of medium-to-strong Bronsted sites of Al-substituted mesoporous silicate is similar to the strength of corresponding sites in HY. Desorption of ammonia from Lewis sites indicates the existence of a broad acid-site distribution ranging in strength from weak to strong. In contrast, Fe-substituted material contains only Lewis sites of weak acid strength. The results indicate that isomorphous substitution is a useful tool to tailor the acidic properties of mesoporous molecular sieves.